JOURNAL O F T H E AMERICAN CHEMICAL SOCIETY Repislered in U.S. Pafcnl Oficc.
@ Copyright, 1969, by the American Chemical Society
VOLUME9 1, NUMBER 20
SEPTEMBER 24, 1969
Physical and Inorganic Chemistry Magnetic Susceptibility Anisotropy, Molecular Quadrupole Moment, Molecular g Values, and the Sign of the Electric Dipole Moment in Methylacetylene R. L. Shoemaker' and W. H. Flygare Contribution from the W. A . Noyes Chemical Laboratory, University of Illinois, Urbana, Illinois. Receiued March 20, 1969 Abstract: The high-field molecular Zeeman effect has been observed for CH3C=--CH,CH,C=CD, and C D 3 4 H under high resolution. An analysis of the data gives the perpendicular molecular g values of g , = +0.00350 f 0.00015 for CH3C=CH, g l = +0.00271 f 0.00015 for CD,(==.--CH,and gl= +0.00367 f 0.00015 for CH3C=CD. The parallel molecular g value is g1I = 0.312 f 0.002 for methylacetylene. The magnetic susceptibility anisotropy is found to be (xL - xI,) = (7.70 i:0.14) X 10-6erg/(G2mole), and the molecular quadrupole moment in CHsC=--CH is Q , , = (f4.82 0.23) x 10-26 esu cm2. The sign of the electric dipole moment for methylacetylene is found to be +CH3CCH-. The elements in paramagnetic susceptibility tensor are x l l p = 9.50 + 0.03 and x L p = 158.41 i: 0.04 in units of 10-6 erg/(G2 mole). The anisotropy of electronic charge distribution is (0 Eta,20) - (01Z,b,*/O) = (33.28 =t0.05) x cm2. Using an estimated bulk magnetic susceptibility, the total magnetic susceptibility tensor elements are determined, giving xL = -28.9 2.1 and x I , = -36.6 2.1 in units of IO+ erg/(G2 mole). The second moments of electronic charge distribution are (OIZ,U,~~O)= (38.72 f 0.30) X cm2 and (OIZ,b,210) = (5.44 f 0.25) x 10-16 cm2. The a axis is the symmetry axis of the molecule. The D-C bond deuterium nuclear quadrupole coupling constant is determined for CD3C=CH to be XD(C-Dbond) = +(I76 15) kHz. The results are discussed and comparisons are made with other similar molecules.
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he rotational Zeeman effect can lead to new information about the ground- and excited-state electronic structureof molecules. The first-order Zeeman effect leads to the measurement of the molecular g values2* and the second-order Zeeman effect leads to the measurement of the anisotropy of the magnetic susceptibility.2b The measurement of these quantities leads directly to an experimental determination of the molecular quadrupole moments and the second moments of the electronic charge distribution. By measuring the molecular g values in two different center of mass systems (isotopic species), the sign of the electric dipole moment can be obtained. Methylacetylene is a particularly suitable molecule for the study of the molecular Zeeman effect and an in-
vestigation of the electronic structure. The microwave spectra of the parent CH,C=CH and all isotopic species are k n ~ w n , and ~ , ~a complete substitutional structure for the molecule is available.6 The first-order molecular Zeeman effect in methylacetylene has been observed previously in fields up to 10,000 G by Cox and Gordy.' They were unable to measure the very small g value perpendicular to the symmetry axis, g,, and they gave values for the g value par0.006 for allel to the symmetry axis of lglll = 0.298 C H 3 C r C H and lglll = 0.310 f 0.01 for CH3C=CD. We have reexamined the molecular Zeeman effect in methylacetylene under high resolution in magnetic fields up to 30,000 G . The signs and magnitudes of gll and g, are obtained for CH3CCH, C H X C D , and CD3CCH,
(1) National Science Foundation Predoctoral Fellow. (2) (a) J. R. Eshbach and M. W. P. Strandberg, Phys, Rev., 85, 24 (1952); (b) W. Hiitnner and W. H. Flygare, J . Chem. Phys., 47, 4137 (1967). (3) W. Huttner, H. K. Lo, and W. H. Flygare, ibid., 48, 1206(1968).
(4) R. Trambarulo and W. Gordy, ibid., 18, 1613 (1950). (5) L. F. Thomas, E. I. Sherrard, and J. Sheridan, Trans. Faraday SOC.,51, 619 (1955). (6) C. C . Costain, J . Chem. Phys., 29, 864 (1958). (7) J. T. Cox and W. Gordy, Phys. ReG., 101, 1298 (1956).
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5417
5418 as well as the value of the magnetic susceptibility anisotropy, (xl. - xi,), in methylacetylene. These parameters are then combined to yield the molecular quadrupole moment and the sign of the electric dipole moment for methylacetylene. Finally, by estimating the bulk magnetic susceptibility from other data, the second moments of the charge distribution and the elements of the magnetic susceptibility tensor for methylacetylene are obtained. We have also determined the deuterium nuclear quadrupole coupling constant in CD3C=CH in the course of this work. The results obtained in this work are compared with similar results in other molecules.
Experimental Data and Analysis
A sample of methylacetylene was obtained from K and K Laboratories. The CH3C=CD was obtained from Volk Radiochemical Co., and the CD3C=CH from Merck Sharp and Dohme. The microwave spectrometer and electromagnet used in this work have been described elsewhere.8 The spectrometer is a relatively standard high-resolution instrument. The electromagnet gives homogeneous magnetic fields of up to 30,000 G over an area of 2 X 72 in. with a gap spacing of 0.6 in., and fields of up to 26,000 G with a gap spacing of 1.1 in. The energy level expression for a rotating molecule in a magnetic field is given in eq 28 of Huttner and Flygare.?b For a symmetric rotor like methylacetylene, this expression simplifies to
The xD along the symmetry axis in D3CC=CH has not been measured up to this time. Therefore, we have J = 1 transition in observed the zero-field J = 0 D3CC=CH in a large L-band absorption cell" in order to determine the value of xD in the D3CC=CH molecule. The theory of the nuclear quadrupole coupling with three equivalent Zl = Z2 = I 3 = 1 spin nuclei has been given previously.12 We observed the J = 0 4 J = 1 transition under high resolution and achieved a partially resolved multiplet. The unresolved spectra were fit by adding Lorentzian line shapes to yield the value of xD along the symmetry axis in D3CC=CH. The result was
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X ~ ( c 3axis)
=
(55
* 5 ) kHz
(3) If we assume that the C-D bond is cylindrically symmetric, we can find the value of xD along the C-D bond axis from f
where 0 is the C-C-H angle which is known6 to be 110" 14'. The results give xD along the C-D bond in D~CCECH of =
(176
* 15) kHz
(5) We now know the values of xD(CS axis) in both D3CC=CH and H3CC=CH. However, in both molecules, the spins are uncoupled (as eq 2 is satisfied) to within the limits of our resolution at 25,000 G. We have found appreciable coupling remaining in H3C@l4N at 25,000 G where XN = -4212 kHz.13 Thus, as in both D3CC=CH and H3CC=CD is a factor of 20 smaller in magnitude than xN in H3CC=N, we feel confident in using the uncoupled basis and firstorder theory. However, there are still diagonal corrections to eq 1 due to the nonzero values of xD along the symmetry axes. The diagonal correction to eq 1 in H3CC=CD is XD
zD
E(J,K,M) =
J , K,and M are the rotational quantum numbers, Eo is the zero-field rigid rotor energy, H is the magnetic field, and po is the nuclear magneton. This expression does not include the effects of the hydrogen nuclear magnetic moments. These effects are negligible since the coupling of the nuclear spins to the molecular rotation is unobservably small, and the anisotropies in the nuclear magnetic shieldings lead to effects of less than 1 kHz.2b For C D 3 C r C H and CH3C=CD, the effects of nuclear quadrupole coupling must be considered. The deuterium nucleus will be uncoupled from the molecular rotation if9 XD
